Refrigeration and freezer systems, especially of the home appliance type, provide cooled air to food storage enclosures. Air is blown over heat exchangers which extract heat from the air to produce the cooled air. The heat exchangers generally operate on the known cooling effect provided by gas that is expanded in a closed circuit, i.e., the refrigeration cycle. In order to be expanded, the gas is first compressed in a compressor. As is known, the efficiency of a system can be enhanced by reducing the amount of frost that builds up on the heat exchanger. Present new systems generally are of a self-defrosting type, i.e., they employ a heater specially positioned and controlled to provide sufficient heat to the enclosure to cause melting of frost build-up on the heat exchanger. Such defrost heaters are controlled by various defrost cycle algorithms and configurations.
Refrigeration and freezer systems have two general cycles or modes, a cooling cycle or mode and a defrost cycle or mode. During the cooling cycle, the compressor is connected to line voltage and the compressor is cycled on and off by means of a thermostat. The compressor is actually run only when the enclosure warms to a preselected temperature. During the defrost cycle, the compressor is disconnected from line voltage and instead, a defrost heater is connected to line voltage. The defrost heater is turned off by means of a temperature responsive switch, after the build-up frost has been melted away.
According to the prior art, operation of the compressor and defrost heater is controlled using a defrost cycle controller generally by one of several techniques referred to herein as real or straight time, cumulative time and variable time. According to real time, the connection of the system to line voltage is monitored and the interval between defrost cycles is based on a fixed interval of real time. Cumulative time involves monitoring the cumulative time a compressor is run during a cooling cycle with the interval between defrost cycles varied based on the cumulative time the compressor is run. Variable time involves allowing for variable intervals between defrost cycles by monitoring both cumulative compressor run time as well as continuous compressor run time and defrost cycle length. The interval between defrost cycles then is based more closely on the need for defrosting.
Defrost systems as described above use more energy than is needed to prevent excessive frost build-up which prevents efficient cooling.